The invention relates to a method for digital transmission of messages in a digital radio relay system which utilizes a superframe, so that additional information can be transmitted with the digital signals.
In a digital radio realy system which has two terminals and one or more repeater stations, additional bits are inserted into the digital signal which is transmitted from the transmitting terminal. These additonal bits are then removed at the receiving terminal. These additional bits consist of frame alignment bits, parity bits and service bits, which are arranged according to the superframe used. They can be arranged either in individual bits or in bytes. The frame alignment bit enables the receivers in the repeater terminals to synchronize themselves with the superframe and to evaluate parity and service bits. The parity bits monitor the transmission quality . The service bits transmit alarm messsages, the RF channel code, and the digital service signals for service calls, long distance monitoring and switching commands.
A scrambling of the digital signals is connected with the superframe. In the transmitter a periodic bit pattern is generated synchronous to the superframe and added to the digital signal so that a scrambled digital signal is generated. In the receiver the same scrambler sequence is generated and added to the scrambled digital signal so the original digital signal is retrieved. In both cases the addition is carried out as a modulo-2-addition (binary addition). The generator for the scrambler sequence is usually a feedback shift-register which is reset periodically, such as at the beginning of the superframe.
A method for inserting additional bits into a superframe is described in the TELCOM Report 5 (1982) 2, at pages 138-144. In this method the superframe contains two bytes, with each byte having k additional bits between which l information bits are located. However, in this method, the possibility exists that a frame alignment signal can be simulated by service bits in the second byte. Also, during the transmission of periodic patterns whose period divisors are 2:1, the frame alignment signal can be simulated at any location in the superframe. However, by eliminating these simulations the parameter selection of the superframe and the scrambling are reduced.